Electrically conductive components

ABSTRACT

Metal elements, usually of refractory metal, for electrical devices such as filaments for incandescent lamps, discharge lamp cathodes, grids or heaters for thermionic valves and radiation elements for electric heaters are made by forming an etch resist pattern on a metal web, etching the web, removing the resist and bending the etched product into compact form. The element produced has a substantially rectangular section which provides a high surface area to cross-sectional area ratio, leading to high thermal and light emission efficiency.

United States Patent 1191 Vause Jan. 29, 1974 ELECTRICALLY CONDUCTIVECOMPONENTS [75] Inventor: Arthur Samuel Vause, London,

England [73] Assignee: Thorn Electrical Industries, Limited, London,England 22 Filed: July 26,197]

21 Appl.No.: 165,516

[30] Foreign Application Priority Data Dec. 15, 1970 Great Britain59,567/70 [52] US. Cl 316/17, 29/25.17, 29/624, 313/315 [51] Int. Cl.H01j 9/02 [58] Field of Search 29/625-627, 602,

[56] References Cited UNITED STATES PATENTS 2,666,254 1/1954 Eisler29/602 2,722,621 11/1955 Schenan 29/602 2,773,239 12/1956 Parker 29/602UX 3,089,106 5/1963 Soaty 29/602 3,216,089 ll/l965 Dettman 29/630 B1,013,157 5/1909 Hadaway 1,674,488 6/1928 Tang 1,697,607 l/1929Maxson....

2,655,582 10/1953 Kirby 338/207 X OTHER PUBLICATIONS Durand, MetalsHandbook, Vol. 3, Chemical Machining, 1967, pp. 240244.

Primary Examiner-Charles W. Lanham Assistant Examiner-Joseph A.Walkowski Attorney, Agent, or Firm-Robert F. OConnell; Dike, Bronstein,Roberts & Cushman [57] ABSTRACT Metal elements, usually of refractorymetal, for electrical devices such as filaments for incandescent lamps,discharge lamp cathodes, grids or heaters for thermionic valves andradiation elements for electric heaters are made by forming an etchresist pattern on a metal web, etching the web, removing the resist andbending the etched product into compact form. The element produced has asubstantially rectangular section which provides a high surface area tocrosssectional area ratio, leading to high thermal and light emissionefficiency.

' 7 Claims, 11 Drawing Figures ymminmzs 1914 3,788,721

SHEET 1 BF 3 ELECTRICALLY CONDUCTIVE COMPONENTS The present inventionrelates to electrically conductive components for electrical devices andmethods of making them. Examples of such components include filamentsfor electric incandescent lamps, discharge lamp cathodes, radiationelements for electric heaters and grids and heaters for'thermionicvalves.

In accordance with the present invention there is provided a method ofmaking an electrically conductive component for an electrical devicewhich comprises etching away a metal web having etch-resistant materialon both faces thereof disposed in such manner as to protect the portionscorresponding to the component it is desired to produce, with at leastone face of the web having the etch-resistant material disposed in apattern correspondingto the components it is desired to produce, andremoving the etch-resistant material from both faces pf the componentthus produced. Initially the portions of the component produced lie inthe simple surface of the workpiece web. In order to improve theoperating effeciency of thermal or incandescent elements made inaccordance with the invention, the component produce by etching the webcan be bent into compact form in which portions of the element arebrought closer together. Preferably, both faces of the web have theetch-resistant material disposed in a pattern corresponding to thecomponent it is desired to produce, the patterns being in overlyingrelationship to one another.

The method of the invention is particularly suitable for automatedmanufacture of electrical components, and may be applied to themanufacture of components from metal webs in the form of foil, ribbon orsheet whereby continuous or batch production can be employed as thecircumstances require.

A further advantage is that changes in the shape of the componentproduced can readily be made merely by applying a different pattern ofetch-resistant material. It is preferred to employ a photographic methodfor providing the etch resistant pattern or patterns on the web, wherebydesign changes can be made by amendment of the master drawing of thepattern, or substitution of a different drawing.

Where the component is to be used as a thermal or incandescent elementit will normally be produced in elongate form, and be made of arefractory metal. It will be appreciated that the component produced byetching a web will be a rectangular (which term is intended to includesquare) cross-section. Thermal or incandescent elements produced inaccordance with the invention have the advantage, as compared with known.elements of circular section wire, of having a greater ratio of surfacearea .to cross-sectional area, thereby providing proportionalincrease inemmitted radiation and therefore in light output.

For example, comparing equal cross-sectional areas in circular andrectangular forms the surface of a rectangular section (of 4:1 sideratio) exceeds that of the circular section by more than 40percent.

In a further aspect of the invention there is provided an electricaldevice having a heat or light emissive element of substantiallyrectangular section.

The invention also provides an electrically conductive component for anelectrical device comprising an elongate substantially rectangularsection member of refractory metal bent into a compact form.

Where the component is to be employed as a thermal or incandescentelement it is usually bent so as to have a plurality of sectionsinclined to the general direction in which the filament extends.Sections of the element may be parallel to one another and perpendicularto the axis of the filament.

The present invention will now be more fully described, by way ofexample only, with reference to the accompanying drawings in which:

FIGS. 1, 2 and 3 show tungsten webs having lamp filaments definedtherein by surrounding etched away regions;

FIGS. la and 1b show the filament of FIG. 1 bent into alternativecompact forms;

FIGS. 2a and 3a shown the filaments of FIGS. 2 and 3 respectively bentinto compact forms;

FIG. 3b shows fragmentary views of the filament of FIG. 3a provided withtabs for engaging filament supports;

FIGS. 4 and 5 show lamp filaments produced by etching a tungsten web;and

FIG. 4a shows the filament of FIG. 4 bent into compact form.

In an example of the method of the invention, tungsten foil of 0.005inches thickness is initially degreased in liquid or vapour phase withfor example trichlorethylene. The foil is dipped in a solution ofphoto-resist material for example Kodak I(.M.E.R. diluted with an equalpart of K.M.E.R. thinner and with dipping and withdrawl being carried ata controlled rate of 6 in/min. After coating, the foil is allowed to dryfor 5 minutes and is then stored at 120C for 10 minutes.

A twin pair of mask profiles corresponding to the component it isdesired to produce, for example a lamp filament as shown in any Figureof the accompanying drawings, is placed in intimate contact with eachopposing face of the resist-coated foil. The profiles, which may besealed photographic images, may be prepared by scaling-down by aphotographic method a sharply defined and accurately dimensioned masterdrawing.

The foil with the mask profiles is irradiated with ultraviolet richlight from fluorescent lamp, high intensity metal vapour dischargelamps, carbon arcs or pulsed xenon lamps. Pulsed xenon lamps may bepreferred as the exposure time is short and this helps to reduceundercut at the subsequently etched surfaces.

The exposed photo-resist is developed in xylol, typically for between Ito 3 minutes. After development the photo-resist image is washed inwater and baked for 10 minutes at 120C.

The etching step may be carried out chemically or electrochemically(deplating). We prefer chemical etching with a mixed agneous solution ofpotassium ferricyanide, e.g. 48 oz. per gallon, and sodium hydroxide,eg, 7 oz. per gallon, at C for 10 minutes, and with this etching methodwe have found that very little undercut is obtained. The etchantsolution may be spray applied.

The photo-resist can be removed from both sides of the componentproduced by mechanical treatment such as scrubbing or by washing.

It will be appreciated that the patterns of etch resist material can beprovided on the foil by known methods other than photo-resisttechniques.

We have found that tungsten sheet materials of thicknesses down to 0.001in. can be satisfactorily processed by the method described above and,where the component is a lamp filament having sections inclined to thegeneral direction in which the filament extends, with a separationbetween adjacents'ections as little as 0.0005

Referring to FIG. 1, a lamp filament 1 is defined in a piece of tungstenfoil 2 by surrounding etched away regions of the foil, the whole beingproduced by the method described above. Before use, the ends of thefilament l are severed from the remainder of the foil 2.

FIG. 1a shows one way in which the filament 1 can be bent into compactform to give thermal operating efficiency be reducing cooling losseswhen employed as a filament in a gas-containing incandescent lamp.

In FIG. lb the filament 1 is bent into a compact form effecting acompromise between thermal operating efficiency and increasedsourcebrightness.

Whereas known methods of filament manufacture require the manufacturerto hold large stocks of tungsten wires of varying diameters and stocksof winding mandrels, the method of the invention avoids the need formandrels and offers flexibility in the choice of dimensions of thefilament which can be obtained from tungsten sheet of a singlethickness, since the width of the filament can be varied at will.Furthermore, the cost of providing and maintaining high speed precisionmachinery required for known filament winding techniques is avoided. Inlamp manufacture, the cost of shaping filament tails and positioningthem relative to the filament axes is frequently greater than the costof the filament itself and requires precision jigging or skilledoperations. In the method of the invention the filaments can be providedwith tails of any desired profile more simply.

The filaments of the invention can be produced with integral fuseprotection sections at their ends. The fuse section can be ofreduced-cross-section appropriate to the circuit protection requirementsand at a distance from the incandescent region of the filament suited tothe particular lamp design.

In known coil-wound filaments, filament failure through burn-out hasbeen attributed to the presence of severe thermal gradients betwenadjacent turns. To reduce the likelihood of such failure the practice ofwinding filaments with varying pitch between adjacent turns has beenadvocated to even out the temperature over the whole of the filamentlength. FIG. 2 illustrates how such variation can be applied in thepresent invention, where a filament 3 has sections inclined to thegeneral direction in which the filament 3 extends and the spacing ofadjacent sections near the ends of the filament 3 is smaller than thespacing intermediate the ends. FIG. 2a shows how varying pitch betweenthe sections results when the filament 3 is bent into compact form.

In known methods in which wire is wound on a mandrel, the space occupiedby the mandrel results in a relatively large hollow core in the formedfilament. The invention avoids the need for a mandrel, allowing greaterscope in designing the filament to achieve optimum performance. FIGS. 3and 3a illustrate how the separation between portions of a filament 6can be minimised within the total space occupied. In FIG. 3a thefilament 6 is bent to achieve a close approximation to a solid lightsource.

In forming a filament by the method of the invention, the etch-resistantmaterial may be so disposed as to produce a filament which is-elongateand has regularly spaced tab portions projecting laterally therefrom,the tab in use serving to engage supports for the filament. In FIG. 3bthere are shown tab portions 7 which can be employed as support-engagingportions for the filament 6. By employing tab portions 7, undesirablemechanical stresses and localised cooling, which could occur where thesupports contact the filament, are avoided.

FIGS. 4 and 4a show respectively a lamp filament 8 of the invention whenthe planar form and when bent into compact form. The form of filamentshown in FIGS. 4 and 4a provides a basis from which a closeapproximation to uniformly bright spherical source, suited to manyoptical applications, may be achieved.

FIG. shows a lamp filament 9 suitable for digital display formed by themethod of the invention. The

- lamp comprises seven segments having individual integral terminals 11through which the segments can be energised singly or in combination.

In lamp manufacture it is usually advantageous to employ filaments ofmetal having selected crystalline characteristics, as for example in gasfilled high performance lamps where filaments of tungsten havingwell-developed and interlocking tungsten crystals are desirable. In themethod of the invention, the crystal structure of the initial metal webcan be determined by etching the surface of the web and identifying thecrystal boundaries. The regions of the web from which it is desired toform the filaments can then be selected, whereby it is possible toproduce filaments having any pre-selected crystal structure.

The rectangular section of lamp filaments in accordance with theinvention facilitates their attachment to components within lamps forexample leads, shields, refectors and insulating bridges, since they arestronger than conventional circular section filaments, and can be moreprecisely located. In particular their use in connection with thefilament supports described in our co-pending United Kingdom PatentApplication No. 18294/ offers significant advantages.

The use of filaments in accordance with the invention may simplify lampmanufacture where rectangular section tails of the lamps are employed aslead-in wires and are sealed directly through the envelope wall.

Where fused silica envelopes are employed the filaments may haveportions of their tails subjected to rolling or chemical etching toreduce their thickness and cross-section to values permitting them to besealed hermetically through the envelope wall. Filaments in accordancewith the invention operating at temperatures of l,500C and upwards, offor example tantalum molybdenum or tungsten, may be sealed directlythrough glass or glass-ceramic envelopes of matching expansion, forexample boro and alumina silicate glasses.

Heater filaments for valves or low temperature lamp filaments inaccordance with the invention, of for example nichrome or nickel ironalloy, can be sealed directly through soft glass (lime soda) envelopes.

Although the invention has been described mainly as applied toincandescent lamp filaments, it will be apparent that the invention canbe readily applied to discharge lamp electrodes for low and highpressure lamps, heating elements for domestic electrical heaters,heaters and grids for thermionic valves and cathode ray tubes.

In particular, the advantages of the invention, where applied toelectrodes for low pressure fluorescent tubs,

where flat tape-like electrodes of close pitch interweaving constructioncan be employed, and for high pressure discharge lamps, where thin sheetenclosures hav ing precisely dimensioned and spaced aperturessurrounding an emissive material core can be employed, are comparable tothose obtained with incadescent filaments lamps.

I claim: 1. A method of manufacturing a gas-filled electric incandescentlamp comprising the steps of:

providing a lamp envelope; preparing a filament therefor by a methodwhich includes the steps of providing etch resistant material coatingson both faces of a refractory metal web, at least one of said coatingshaving the form of a pattern corresponding to the component it isdesired to make; etching said web to etch away metal from areas thereofunprotected by said coatings; and removing said coatings; positioningsaid filament within said envelope; sealing said filament within saidenvelope; and gasfilling said envelope.

2. A method according to claim 1 wherein both of said coatings have theform of a pattern corresponding to the component it is desired to make,said patterns being disposed in overlying relationship.

3. A method according to claim 1 wherein said pattern coating isprovided on said web by a photographic method.

4. A method according to claim 1 wherein said pattern has an elongateform with regularly spaced tab portions projecting laterally therefrom.

5. A method according to claim 1 wherein said web is of tungsten.

6. A method according to claim 5 wherein said web is etched with asolution of potassium ferrocyanide and sodium hydroxide.

7. A method according to claim 1 and further including the step ofbending said etched web to bring closely adjacent one another portionsof said etched web which are spaced apart along said etched web, wherebysaid etched web is brought into compact form before being positioned insaid envelope.

* =l =l l

2. A method according to claim 1 wherein both of said coatings have theform of a pattern corresponding to the component it is desired to make,said patterns being disposed in overlying relationship.
 3. A methodaccording to claim 1 wherein said pattern coating is provided on saidweb by a photographic method.
 4. A method according to claim 1 whereinsaid pattern has an elongate form with regularly spaced tab portionsprojecting laterally therefrom.
 5. A method according to claim 1 whereinsaid web is of tungsten.
 6. A method according to claim 5 wherein saidweb is etched with a solution of potassium ferrocyanide and sodiumhydroxide.
 7. A method according to claim 1 and further including thestep of bending said etched web to bring closely adjacent one anotherportions of said etched web which are spaced apart along said etchedweb, whereby said etched web is brought into compact form before beingpositioned in said envelope.